Various optical elements composed of optical glass such as optical pickup lenses for optical discs and image pickup lenses to be installed in mobile phones are widely utilized. Recently, demands for such the optical elements are rapidly enlarged and rising in the production efficiency and lowering in the cost are required. Consequently, a press molding method has been widely applied for producing such the optical elements. According to the press molding method, the production process has been reduced compared to usual polishing method so that the optical elements can be produced in shorter time and at lower cost.
The press molding method is classified into a re-heating method and a direct press method. In the re-heating method, a gob preform or a polished preform in a shape which is resembles in the final shape of the product is prepared and the preform is re-heated at a temperature not lower than the yield point of the glass and pressed with a pair of upper and lower molds which are heated to obtain the final shape of the product. In the direct press method, a molten glass drop is directly dropped from a glass melting tank onto a heated mold, and the molten glass drop is pressed into the shape of the final product.
In the both methods, the mold is necessarily heated at a temperature near or higher than the glass transition temperature Tg of the optical glass to be formed, when the glass is pressed with the mold. Consequently, surface oxidation or changing in the metal composition of the mold tends to be caused when the glass transition temperature Tg of the optical glass is higher so that the lifetime of the mold is shortened and the cost is raised. A method for inhibiting such the deterioration of the mold can be considered, in which the molding is performed under an atmosphere of inactive gas such as nitrogen. However, the production cost is raised in such the method since the molding apparatus is made complicate for controlling the atmosphere and running cost of the inactive gas is necessary.
Therefore, glass with a glass transition temperature Tg as low as possible is preferable as optical glass to be used in the press molding method. Moreover, it is preferable from the viewpoint of working environment at the production that PbO, As2O3, TeO2, Sb2O3 and fluoride components are not contained in the optical glass.
From the above viewpoints, some kinds of glass having medium refractive index, low dispersion and low glass transition temperature Tg are proposed; cf. Patent Publications 1 to 5, for example.    Patent Publication 1: JP-A 2004-217513    Patent Publication 2: JP-A 2007-145613    Patent Publication 3: JP-A Hei 2-124743    Patent Publication 4: JP-A 2002-211949    Patent Publication 5: JP-A 2005-53749
However, component ratios of the optical glasses described in the above Patent Publications 1 to 5 are decided from viewpoint of lowering the glass transition temperature Tg as the most important matter and such the glasses have a drawback that the toughness is low (fragile). Therefore, cracks are easily caused by a shock given on the occasion of press molding or in the post-processing such as centering carried out after the press molding process.